Epoxy resins have a broad range of physical characteristics and, because of this, they are used in many industrial applications. Epoxy resins have at least one epoxy group which can be converted to a thermoset form having desirable properties. These epoxy groups may be cured by the use of a catalyst or a curing agent. The curing may be accelerated by the addition of small but effective amounts of accelerating agents.
There are many different types of curing agents which may be utilized. One class of curing agents which can be used are the anhydrides. The most common of these anhydride curing agents are the difunctional compounds such as maleic anhydride, phthalic anhydride and the like, as well as tetrafunctional materials such as pyromellitic dianhydride.
Ureas and substituted ureas have been utilized as epoxy curing agents, co-curing agents and curing accelerators. These urea and substituted urea compounds have been disclosed in U.S. Pat. Nos. 3,294,749; 2,713,569; 3,386,956; 3,386,955; 2,855,372; and 3,639,338.
Compounds having a single terminal ureido group have been disclosed in U.S. Pat. Nos. 2,145,242 and 3,965,072.
Epoxy resins which are used for casting, imbedding or encapsulating must have the ability to withstand repeated cycles of high and low temperatures without cracking. As the temperature decreases, the stress increases due to shrinkage since the lowering of the temperature reduces the ability of the resin to flow and relieve the stress.
Anhydride cured resins are most useful in applications requiring high heat deflection. However, anhydride cured resins are generally brittle and, thus, have a low resistance to thermal shock. Diluents and modifiers do improve the resistance to thermal shock; however, these materials adversely affect the heat deflection properties as shown in May and Tanaka, EPOXY RESINS, New York, 1973, p. 299. Similarly, plastisizers have not been widely used with epoxy resins because most are incompatible with the cured resins.